1. Field
The present invention relates to an organic electroluminescence device, a method of manufacturing an organic electroluminescence device, a display apparatus and an illumination apparatus.
2. Description of the Related Art
Research into organic-material utilized devices, such as organic electroluminescence devices (hereinafter referred to as organic EL devices too) and organic-semiconductor utilized transistors, has been actively conducted. As to the organic EL devices in particular, it has been expected to develop them for use in large-area and full-color display devices of solid-state light emission type or for illumination use as low-priced large-area surface light sources. In general an organic electroluminescence device is made up of organic layers, including a light emitting layer, and a pair of electrodes between which the organic layers are sandwiched. When a voltage is applied to such an organic electroluminescence device, electrons from the negative electrode and holes from the positive electrode are injected into the light emitting layer. These electrons and holes are recombined in the light emitting layer, and luminescence is produced by energy's being emitted in the form of light at the time of recovery of the energy level from the conduction band to the valence band.
In view of luminous efficiency and lifetime of an organic EL device, the light emitting layer of an organic EL device is usually obtained through film formation by vapor deposition of ingredients to constitute such a light emitting layer. Organic electroluminescence devices of a type that vapor deposition is utilized in the making thereof (hereinafter referred to as deposition-type organic EL devices too) have been in practical uses as light emitting sources for displays of cellular phones, television sets and the like.
In recent years, however, further reduction in manufacturing costs of organic electroluminescence devices has been required in both the case of using them for display devices and the case of using them for illumination apparatus.
For instance, the organic electroluminescence devices that are made by using organic solvents in which ingredients to constitute light emitting layers are dissolved or dispersed and coating them (which are also referred to as coating-type organic EL devices hereinafter) are announced in U.S. Pat. No. 7,453,200 and JP Patent 3541294. Although the vapor deposition method in the manufacturing of deposition-type organic EL devices is great in installation cost of vapor-deposition apparatus and energy cost at the time of vapor deposition of ingredients, these costs can be omitted in the manufacturing of coating-type organic EL devices, and besides, the coating-type organic EL devices have an advantage that it is easy to manufacture large-screen organic EL displays also.
The coating-type organic EL devices disclosed in U.S. Pat. No. 7,453,200 and JP Patent 3541294, however, have problems that, because each of their light emitting layers is formed of a single layer, they are insufficient in luminous efficiency, durability and color reproducibility when two or more kinds of light emitting materials are incorporated into each of their light emitting layers for the purpose of obtaining a desired color of light emission (e.g. white-light emission in view of the use in illumination apparatus).
On the other hand, JP-A-2009-152435 discloses the organic EL device having multiple light emitting layers, and therein examples of the device whose light emitting layers each are formed by use of a spin coating method are disclosed. However, the coating in accordance with a spin coating method tends to cause damage to the underlying layer, and hence the organic EL device occasionally underwent deterioration in luminous efficiency and durability.
In addition, JP-A-2007-48732 also discloses the organic EL device having multiple light emitting layers, and therein an example of the illumination apparatus having the organic EL devices whose multiple light emitting layers each are formed in accordance with an inkjet method is disclosed. In this organic EL device, each light emitting layer is cured through the use of azobis(isobutyronitrile) as a polymerization catalyst, and thereby multilayer coating is fulfilled. According to such a method, however, the residue structure of the polymerization catalyst is left in each light emitting layer, which caused a problem of exacerbating EL properties, especially durability of the device.